UVP-DUO's high temporal resolution help understanding turbulent free-surface vortex flows using a Taylor- Couette flow analogy

22 May 2024 by
UVP-DUO's high temporal resolution help understanding turbulent free-surface vortex flows using a Taylor- Couette flow analogy
Met-Flow SA

Unveiling the Dynamics of Free-Surface Vortex Flows: The Role of UVP-DUO


Understanding the intricate behaviors of free-surface vortex (FSV) flows remains a cornerstone in fluid dynamics, revealing patterns that closely resemble the well-studied Taylor-Couette flow (TCF). Recent advancements in measurement techniques have significantly enhanced our ability to observe and analyze these complex flow systems. A pivotal instrument in these studies is the UVP-DUO, a sophisticated Ultrasonic Doppler Velocity Profiling system, whose high temporal resolution of 9ms (111Hz) has proven crucial for capturing the dynamic features of turbulent FSVs.


Experimental Overview


In the study "Understanding turbulent free-surface vortex flows using a Taylor-Couette flow analogy," Mulligan et al. explored the turbulent FSV by employing an analogy with TCF, which is characterized by successive instability phases and distinct vortex formations. The research aimed to draw parallels between the secondary flow fields of both systems, particularly focusing on the time-dependent "Taylor-like" vortices observed in the FSV.


Configuration and Role of UVP-DUO


The UVP-DUO system was configured to facilitate detailed two-dimensional velocity measurements within the FSV. Specifically, a 7 × 7 array of ultrasound transducers was aligned along the radial (r) and axial (z) directions, passing through a semi-cross section of the vortex core. This setup allowed for the precise mapping of velocity vectors and vorticity contours, capturing the intricate dynamics of the vortex flow with high temporal resolution.


Insights from High Temporal Resolution


The UVP-DUO's ability to record velocity profiles at an impressive rate of 111Hz (every 9ms) was instrumental in detecting the fine-scale, transient structures within the FSV. These high-frequency measurements enabled the researchers to identify and analyze the "Taylor-like" vortices, which are indicative of rotational instabilities analogous to those in the TCF system.


Key Findings


The experimental results highlighted several critical aspects of the FSV dynamics:


1. **Secondary Flow Field Behavior**: The UVP-DUO measurements revealed distinct, time-dependent vortices in the secondary flow field of the FSV, akin to the Taylor vortices observed in TCF. These vortices are a direct consequence of centrifugal instabilities, which occur when the inward radial pressure gradient fails to counteract the outward inertia of fluid particles  .


2. **Energy Transfer Mechanisms**: Unlike the TCF, where mechanical energy is introduced via rotating cylinders, the FSV derives its rotational energy from the shear-driven circulation field resulting from continuous fluid inflow. This flow induces similar centrifugal instabilities, reinforcing the analogy between the two systems .


3. **Vortex Structure and Stability**: The study proposed that the free-surface vortex can be visualized as a 'virtual cylinder', where the inward flowing fluid maintains the rotation of this cylinder, thus stabilizing the vortex structure. This conceptual framework facilitated a deeper understanding of the various stability modes of the FSV, from stable to chaotic wavy modes  .


Conclusion


The UVP-DUO system, with its high temporal resolution, has significantly advanced our understanding of free-surface vortex flows by enabling the detailed observation of transient vortex structures and their stability mechanisms. This study not only underscores the importance of high-resolution measurement tools in fluid dynamics research but also provides a robust framework for further investigations into complex vortex behaviors using the Taylor-Couette analogy.


By leveraging the capabilities of UVP-DUO, researchers can continue to unravel the mysteries of turbulent flows, paving the way for innovations in various applications, from environmental fluid dynamics to industrial mixing processes.


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References


Mulligan, S., De Cesare, G., Casserly, J., & Sherlock, R. (2018). Understanding turbulent free-surface vortex flows using a Taylor-Couette flow analogy. *Scientific Reports*, Nature Publishing Group.   

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